IgG has been purified from monoclonal antibodies B6.2 and B72.3. F(ab')2 fragments, and Fab' fragments of monoclonal antibody B6.2 have also been generated. The IgG and its fragments were radiolabeled with I-125 and I-131 without loss of immunoreactivity and were injected into athymic mice bearing human mammary tumor transplants or human colon carcinomas. The radiolabeled B6.2 antibody localized in the tumor within 24 hours with tumor to tissue ratios rising over a 96 hour period. The F(ab')2 was better than the IgG and gave tumor to liver and spleen ratios of 15 to 20:1, and tumor to muscle and brain ratios of 50 to 110:1. No localization was observed in mice bearing human melanomas, or with radiolabeled normal murine IgG in mice bearing human mammary tumors or colon carcinomas. The ability of the radiolabeled antibody to localize in mammary and colon tumors was sufficient to give high quality gamma scans of tumor bearing mice. Monoclonal antibody B72.3 was shown to localize human colon cancer xenografts in athymic mice and showed an increase in uptake in the tumor over the first two days post inoculation of the antibody and stayed constant over the 19 day period of study. Monoclonal antibodies B72.3 and B6.2 are being labeled with several isotopes to test for appropriateness for clinical studies for carcinoma localization.